The present invention relates to three-phase brushless motors with half-wave control circuits.
More specifically, the subject of the invention is a three-phase brushless motor including, in known manner, a rotor with an angular array of 2n (n =1, 2, 3 . . . ) permanent magnets with a predetermined pole pitch .tau. =360.degree./2n), in which alternate magnets have opposite polarities but the same angular extent.
In brushless motors of this type which have been produced up to now, the angular extent of each magnet is substantially equal to the pole pitch and each magnet is therefore substantially next to, that is, close to the adjacent magnet or magnets.
FIG. 1 of the appended drawings shows a typical layout of a half-wave control circuit for a three-phase brushless motor. The circuit includes, in known manner, three controlled-conduction devices T1, T2 and T3 (for example, MOSFET transistors) between the two poles of a direct-current voltage supply, each being arranged in series with a respective winding or phase P1, P2 and P3 of the motor. An electronic control circuit CC makes each device T1, T2 and T3 conductive cyclically and in sequence for a period of time corresponding to a conduction angle of 120 electrical degrees, as shown by the first three waveforms indicated W1, W2 and W3 in FIG. 2 of the appended drawings. Each power switch thus conducts for 1/3 of the electrical operating cycle (whilst the others are not conductive).
Since, in three-phase brushless motors of the prior art, the angular extent of each magnet of the rotor is substantially equal to the pole pitch, the electromotive forces induced in the individual phases correspond substantially to the waveforms indicated F1, F2 and F3 in FIG. 2. As can be seen, although one phase is made conductive at a time, an electromotive force is generated in each phase throughout almost 180 electrical degrees. Only a portion corresponding to about 120 electrical degrees of the electromotive force generated in each phase, however, is used. An excess electromotive force is thus generated in each individual phase for about 60 electrical degrees. This clearly represents wastage involving the inefficient usage of the materials constituting the motor, particularly the permanent magnets and the frame, usually of iron, in which the magnetic flux generated by the magnets is enclosed.